selftests/bpf: Test for unaligned flow_dissector ctx access

This patch adds tests for two context fields where unaligned accesses
were not properly rejected.

Note the new macro is similar to the existing narrow_load macro, but we
need a different description and access offset. Combining the two
macros into one is probably doable but I don't think it would help
readability.

vmlinux.h is included in place of bpf.h so we have the definition of
struct bpf_nf_ctx.

Signed-off-by: Paul Chaignon <paul.chaignon@gmail.com>
Tested-by: Eduard Zingerman <eddyz87@gmail.com>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/bf014046ddcf41677fb8b98d150c14027e9fddba.1754039605.git.paul.chaignon@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: pchaigno

tools/testing/selftests/bpf/progs/verifier_ctx.c

@@ -1,10 +1,12 @@
 // SPDX-License-Identifier: GPL-2.0
 /* Converted from tools/testing/selftests/bpf/verifier/ctx.c */
 
-#include <linux/bpf.h>
+#include "vmlinux.h"
 #include <bpf/bpf_helpers.h>
 #include "bpf_misc.h"
 
+#define sizeof_field(TYPE, MEMBER) sizeof((((TYPE *)0)->MEMBER))
+
 SEC("tc")
 __description("context stores via BPF_ATOMIC")
 __failure __msg("BPF_ATOMIC stores into R1 ctx is not allowed")
@@ -243,4 +245,23 @@ narrow_load("sockops", bpf_sock_ops, skb_data);
 narrow_load("sockops", bpf_sock_ops, skb_data_end);
 narrow_load("sockops", bpf_sock_ops, skb_hwtstamp);
 
+#define unaligned_access(type, ctx, field)					\
+	SEC(type)								\
+	__description("unaligned access on field " #field " of " #ctx)		\
+	__failure __msg("invalid bpf_context access")				\
+	__naked void unaligned_ctx_access_##ctx##field(void)			\
+	{									\
+		asm volatile ("							\
+		r1 = *(u%[size] *)(r1 + %[off]);				\
+		r0 = 0;								\
+		exit;"								\
+		:								\
+		: __imm_const(size, sizeof_field(struct ctx, field) * 8),	\
+		  __imm_const(off, offsetof(struct ctx, field) + 1)		\
+		: __clobber_all);						\
+	}
+
+unaligned_access("flow_dissector", __sk_buff, data);
+unaligned_access("netfilter", bpf_nf_ctx, skb);
+
 char _license[] SEC("license") = "GPL";